Throwaway tips of this type are mainly made of sintered hard materials, such as cemented carbide manufactured according to the so-called powder metallurgy which forms a green compact by press-forming raw material powder, places the green compact on a sintering plate, and then receiving and heating the green compact in a sintering furnace to sinter the green compact. Here, in order to press-form a green compact from raw material powder as mentioned above, the die pressing method, which press-forms a green compact by compressing raw material powder filled into used from the viewpoint of process efficiency, as set forth on pages 18 and 19 in “Basis and applications of cemented carbide and sintered hard materials” issued on Feb. 20, 1986 by Suzuki Hishashi in Marujen Co., Ltd. In addition, a plurality of the green compacts formed as mentioned above are placed on one sintering plate in a direction conforming to its shape as compact as possible so that the maximum number of the green compacts may be received in the sintering furnace, and the green compacts are received and sintered in the sintering furnace with a plurality of such sintering plates being superposed.
By the way, as stated in the above literature, it is known that such powder metallurgy causes 15 to 22% of linear shrinkage in, for example, cemented carbide due to sintering of the green compact. Therefore, a dimension difference occurs between the green compact and the throwaway tip after sintering. Particularly in the die pressing method as mentioned above, if the density of the green compact is nonuniform during the press forming, large shrinkage deformation is generated at a portion of low density, which results in deterioration of dimensional accuracy of the sintered body. Conventionally, the above literature also exhibits there are researches for restricting such sintering deformation to the minimum by making the density of one green compact as uniform as possible. Practically, the deformation caused by sintering is restricted to a negligible level by making the dimension difference from the green compact to the throwaway tip after sintering uniform in one green compact as a whole. Incidentally, the conventional throwaway tip whose outer circumferential face (flank face) is made of a sintered skin becomes a so-called M-grade tip, and its dimensional accuracy has inscribed circle allowance of less than ±0.08 mm in a throwaway tip having an inscribed circle of 12.70 mm. If more dimensional accuracy is required, the outer circumference grinding is conducted to form a G-grade tip having an inscribed circle allowance of less than ±0.025 mm.
However, even in such a throwaway tip, there are recently more demands for higher accuracy without increasing its cost. For example, it is required to obtain approximately G-grade accuracy without performing the post-processing, such as the outer circumference grinding, to the throwaway tip which is sintered with a sintered skin as mentioned above. This means high degrees of sintering accuracy for the throwaway tip, which is a sintered product from the green compact. As a result, how to reduce the dimension error caused by the infinitesimal sintering deformation, which is not an issue in the conventional allowance, is now a significant subject.
The present invention has been achieved on the basis of such backgrounds. It is therefore an object of the present invention to provide a method for manufacturing a throwaway tip according to the powder metallurgy, which gives high sintering accuracy to satisfy approximately G-grade accuracy even for the throwaway tip in a sintered state, and to provide an apparatus for aligning of green compacts to the sintering plate, which is very suitable for using this method.